1. Field of the Invention
This invention relates to crosstalk reduction in electrical circuits and in particular to crosstalk reduction in communication circuits.
2. Description of the Related Art
In many communication systems electrical communication signals are conveyed between various equipment used in such systems. Typically, the electrical signals can be interconnected and routed to various equipment located at a central office. The interconnection and rerouting of the electrical signals are implemented with interconnection modules, some of which are called Digital Distributing Frames (DDF).
In a central office location which may contain various communication equipment, a plurality of DDF modules are typically disposed in a housing. Typically, a DDF module comprises several connectors, such as BNC connectors mounted on a printed circuit board. Cables carrying the electrical signals can be connected to the mounted connectors of the various housed DDF modules and to the various equipment at the central location thus interconnecting and rerouting various electrical signals. As is typically used with printed circuit boards, conductors electrically connected to the mounted connectors are formed on the circuit board by well known etching or plating processes. The conductors are typically traced on the surface of the printed circuit board so as to electrically and physically isolate them from other conductors traced on the printed circuit board. Each of the conductors may be associated with a particular electrical circuit and mounted connector. The particular circuit may be represented by conductor traces on the printed circuit board that may or may not be connected to components (e.g., resistors, capacitors, diodes, transformers) mounted on the printed circuit board. Also, each particular electrical circuit has its own electrical ground which is substantially electrically and physically isolated from other electrical grounds and conductor traces on the printed circuit board. The electrical and physical isolation of the conductors help prevent crosstalk between conductors associated with different circuits. Crosstalk is defined as undesired electromagnetic coupling that occurs between proximately located conductors carrying electrical signals.
Current DDF modules support two isolated circuits each of which is electrically connected to a mounted connector. These modules may also have one or two other connectors for monitoring the isolated circuits. Conductor traces connected to the various mounted connectors associated with the isolated circuits may be in close proximity to each other. As a result crosstalk can occur between these traces. However, said crosstalk can be attenuated with electrical shielding structures (e.g., plated vias, ground planes) mounted on the printed circuit board. These structures tend to further isolate the traces, including electrical ground traces, associated with the isolated circuits thereby reducing crosstalk that may occur between these circuits. However, under some circumstances, a resonant condition occurs between isolated circuits which can greatly amplify any crosstalk between such circuits rendering the electrical shielding technique ineffective.
Typically an electrical circuit such as the isolated circuits has a natural frequency which is the frequency at which a circuit oscillates in response to an impulse signal. The isolated circuit may also have circuit loops which are electric paths comprising of various electrical devices including conductors and parasitic elements. A parasitic element is an unintended element that is created as a result of the geometry of an electrical circuit. Thus, a resonant condition occurs when a signal is applied to one or both of the isolated circuits such that the signal frequency matches the natural frequency of unintended circuit loops formed by the parasitic elements. Therefore, a resonant condition occurs at certain frequencies or group of frequencies.
Methods are currently available to reduce the effects of such resonant conditions, but such methods result in the degradation of the signals applied to the isolated circuits. For example, the introduction of loss (connecting a resistor in series with the applied signals) to the isolated circuit can reduce crosstalk but such a method will adversely effect the signal quality (e.g., amplitude, phase and frequency distortions).
Therefore, there exists a need to reduce crosstalk due to resonating isolated circuits which are traced on a printed circuit board.